Component geometry and method for blowout resistant welds

ABSTRACT

In a method for reducing “blow-out” of annular welds for attaching components in a fuel injector, a relief region is formed on radially-facing surfaces of the components. The relief region is adjacent to a press-fit region. The components are then pressed together, and a weld is made in the relief region. A sealed gap is thereby formed in the relief region between the weld and the press-fit region. The sealed gap provides for the expansion of trapped gases that could otherwise “blow out” the liquid weld bead.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional Application Ser.No. 60/691,215 entitled “Blowout Resistant Weld Geometry for LaserWelds,” filed on Jun. 16, 2005, the contents of which are herebyincorporated by reference herein in their entirety.

This application is further related to U.S. patent application Ser. No.11/453,628, entitled “Blowout Resistant Weld Method for Laser Welds forPress-Fit Parts,” filed on the same date as the present application.

FIELD OF THE INVENTION

The present invention relates generally to the field of welding, andmore particularly, to techniques and systems for forming hermetic,blow-out resistant welds between press-fitted components in a fuelinjector assembly.

BACKGROUND OF THE INVENTION

A fuel injector includes a pressure vessel, a valve venting the pressurevessel, and a coil-driven magnetic circuit for driving the valve. Thepressure vessel must not exhibit external fuel leaks during operation.Most fuel injector designs utilize multiple components that are weldedtogether to create the pressure vessel.

Typically, fuel injector pressure vessel components are welded using alaser. A laser has been used successfully to weld joint configurationssuch as lap joints where overlapping surfaces of the components arejointed, butt joints where two components are joined end-to-end withoutoverlap, and fillet joints in which material is removed on abuttingparts to provide room for a weld bead. Lasers are suitable for weldingsmall precision components together dependably and quickly in aproduction environment.

In many applications, the laser beam is held stationary as the part tobe welded is moved or rotated to form the weld. To weld the hermeticpressure vessel used in fuel injectors, the beam is commonly heldstationary while the part is rotated. For a hermetic weld of that typeon tubular components such as those of the fuel injector pressurevessel, the “on” time for the laser beam is greater than the time ittakes the part to make one revolution. The resulting overlap of the weldensures that the weld is hermetic.

One common problem associated with such laser welding on tubularcomponents occurs as the overlap of the weld is formed. Certain weldingconditions and joint designs tend to result in a “blow out” of the weldbead, usually during final overlap of the weld. That “blow out” iscreated by rapidly increasing internal pressure on one side of the weld,due to a sudden rise in temperature related to the welding. The “blowout” occurs most commonly as the weld overlap occurs, although undercertain conditions it is known to occur elsewhere. If an internal regionto either side of the weld joint is undergoing a sufficient pressureincrease, the weld “blow out” occurs when the molten weld pool is unableto resist the forces exerted by the pressure differential. The weld“blows out,” leaving a hole or gap in the weld bead. That hole typicallyleads to an increase in leak-related scrap during the assembly process.

For example, two components may be lap welded together at a continuous“interference fit” or press-fit region. Such welds have been known toexhibit “blow-out” regions at random locations relative as well as inthe overlap. Those “blow-outs” are often at multiple radial locationsthroughout the weld. It has been theorized that in a press fit region,small cavities contain trapped air due to an imperfect surface finish ofthe components pressed together. When laser welding is attempted overthose small cavities, the air inside undergoes a sudden change intemperature and expands. That expansion “blows out” the molten weldpool, leaving behind a void in the weld.

Alternatively, the two parts may be joined without a press fit and withclearance between the facing surfaces. No differential pressure iscreated, and therefore there are virtually no “blow-outs.” That jointdesign, however, has two significant drawbacks when used in a fuelinjector application. First, any weld slag or oxides created by thewelding process can escape from the weld joint into the valve body,creating internal contamination of the fuel injector. Such internalcontamination in a precision device such as a fuel injector can haveundesirable effects. Secondly, many designs require a press fit betweenthe two components for processing reasons.

There is therefore presently a need to provide a method and system forreliably creating a hermetic weld joining tubular components of a fuelinjector. To the inventors' knowledge, no such technique is currentlyavailable.

SUMMARY OF THE INVENTION

One embodiment of the present invention is a method for forming a fuelinjector having a fuel inlet, and fuel outlet and a fuel passagewayextending from the fuel inlet to the fuel outlet along a longitudinalaxis. The method includes the steps of constructing a first fuelinjector component comprising a radially outwardly facing annularsurface; constructing a second fuel injector component comprising aradially inwardly facing annular surface; shaping at least one of theannular surfaces to form, when the surfaces overlap, a non-contactregion having a gap between the surfaces, and a press-fit region wherethe first and second surfaces are in contact, the non-contact and pressfit regions being adjacent; assembling the first and second componentsby overlapping the annular surfaces; and welding the annular surfacestogether in the non-contact region to form an annular weld bead, theweld bead and the contact region bounding a substantially sealed portionof the gap.

The surfaces in the non-contact region may be between 0.005 and 0.025 mmapart. A center of the weld bead may be at least 1 mm from the contactregion.

The first fuel injector component may be a pole piece, while the secondfuel injector component may be a non-magnetic shell.

The step of welding the annular surfaces together may be a laser weldingoperation. The laser welding operation may include the steps of rotatingthe assembled first and second components about a longitudinal axis, andapplying a laser welding beam. More particularly, the laser weldingoperation may include rotating the assembled first and second componentsat 200 RPM; and applying a laser welding beam for 285 milliseconds at850 Watts, then applying the laser welding beam for 45 milliseconds at820 Watts.

The shaping step may form the non-contact region on a longitudinal sideof the press-fit region opposite a valve body of the fuel injector.

Another embodiment of the invention is a fuel injector having a fuelinlet, and fuel outlet and a fuel passageway extending from the fuelinlet to the fuel outlet along a longitudinal axis. The fuel injectorfurther comprises a first fuel injector component comprising a firstcomponent inlet, outlet, and passageway, the first component passagewayextending from the first component inlet to the first component outletalong the longitudinal axis, the first component further comprising aradially outwardly facing exterior surface; a second fuel injectorcomponent comprising a second component inlet, outlet, and passageway,the second component passageway extending from the secon component inletto the second component outlet along the longitudinal axis, the secondcomponent further comprising a radially inwardly facing interiorsurface; the exterior surface of the first component facing the interiorsurface of the second component; an annular weld bead connecting theinterior and exterior surfaces; and an annular press-fit region offsetfrom the annular weld bead in a longitudinal direction, the interior andexterior surfaces being in contact in the press-fit region; the annularweld bead and the annular press fit region delineating a sealed annularclearance region wherein the interior and exterior surfaces are spacedapart.

The annular clearance region of the fuel injector may have a ratio oflength to width greater than 10. The interior and exterior surfaces maybe spaced apart in the annular clearance region between 0.005 and 0.025mm. A center of the weld bead may be at least 1 mm from the press-fitregion. The first fuel injector component may be a pole piece and thesecond fuel injector component may be a non-magnetic shell.

The annular weld bead may be a laser weld bead. The fuel injector mayfurther comprise a fuel injector valve body, the non-contact regionbeing on a longitudinal side of the press-fit region opposite the valvebody.

In yet another embodiment of the invention, a method is provided forassembling a pole piece and a non-magnetic shell of a fuel injector, thepole piece comprising a radially outwardly facing annular surface andthe non-magnetic shell comprising a radially inwardly facing annularsurface. The method includes the steps of forming an annular reliefregion on at least one of the annular surfaces; assembling thenon-magnetic shell and the pole piece with the annular surfacesoverlapping; the first and second surfaces being in contact in apress-fit region, and the first and second surfaces defining a gaptherebetween at the relief region; welding an annular weld bead in therelief region, the weld bead joining the annular surfaces, the weld beadand the press-fit region bounding a substantially sealed portion of thegap.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of a fuel injector according to oneembodiment of the invention.

FIG. 2 is a sectional view of a pole piece and non-magnetic shellassembly according to one embodiment of the invention.

FIG. 3 is an enlargement of the region III-III of FIG. 2.

FIG. 4 is a flow chart depicting a method according to one embodiment ofthe invention.

DESCRIPTION OF THE INVENTION

The inventors have developed a technique and a fuel injector design toaddress the above-described problems in welding fuel injector pressurevessels. A cross-sectional view of a fuel injector 100 according to oneembodiment of the invention is shown in FIG. 1. While the invention isdescribed in connection with that exemplary fuel injector, one skilledin the art will understand that the inventive method and apparatus areapplicable to other fuel injector designs. Embodiments of the inventionmay further be used in other welding applications where weld “blow out”is a concern.

Referring to FIG. 1, a solenoid actuated fuel injector 100 dispenses aquantity of fuel that is to be combusted in an internal combustionengine (not shown). The fuel injector 100 extends along a longitudinalaxis A-A between a first injector end 200A and a second injector end200B, and includes a valve group subassembly 200 and a power groupsubassembly 300. The valve group subassembly 200 performs fluid handlingfunctions, e.g., defining a fuel flow path and prohibiting fuel flowthrough the injector 100. The power group subassembly 300 performselectrical functions, e.g., converting electrical signals to a drivingforce for permitting fuel flow through the injector 100.

The valve group subassembly 200 includes a tube assembly 202 extendingalong the longitudinal axis A-A between the first fuel injector end 200Aand the second fuel injector end 200B. The tube assembly 202 can includeat least an inlet tube 204, a non-magnetic shell 210, and a valve body206. The inlet tube 204 has a first inlet tube end 202A proximate to thefirst fuel injector end 200A. The inlet tube 204 can be flared at theinlet end 202A into a flange 202C to retain an O-ring 10. A second inlettube end 202B of the inlet tube 204 is connected to a first shell end210A of the non-magnetic shell 210. A second shell end 210B of thenon-magnetic shell 210 can be connected to a generally transverse planarsurface of a first valve body end 206A of the valve body 206. A secondvalve body end 206B of the valve body 206 is disposed proximate to thesecond tube assembly end 200B. The inlet tube 204 can be formed by adeep drawing process or by a rolling operation. A separate pole piece208 can be connected to the inlet tube 204 and connected to the firstshell end 210A of the non-magnetic shell 210. The pole piece maycomprise a stainless steel material such as SS 430FR (ASTM A838-00). Thenon-magnetic shell 210 can comprise non-magnetic stainless steel, e.g.,300-series stainless steels such as SS 305 (EN 10088-2), or othermaterials that have similar structural and magnetic properties.

As shown in FIG. 1, inlet tube 204 is attached to pole piece 208 bymeans of welds 205. Formed into the outer surface of pole piece 208 arepole piece shoulders 208A, which, in conjunction with mating shouldersof a bobbin of the coil subassembly, act as positive mounting stops whenthe two subassemblies are assembled together. The length of pole piece208 is fixed whereas the length of the inlet tube 204 can vary accordingto operating requirements of the particular fuel injector design. Byforming inlet tube 204 separately from pole piece 208, different lengthinjectors can be manufactured by using different inlet tube lengthsduring the assembly process. The inlet tube 204 can be attached to thepole piece 208 at an inner circumferential surface of the pole piece208. Alternatively, an integral inlet tube and pole piece can beattached to the inner circumferential surface of a non-magnetic shell210.

An armature assembly 212 is disposed in the tube assembly 202. Thearmature assembly 212 includes a first armature assembly end having aferro-magnetic or armature portion 214 and a second armature assemblyend having a sealing portion. The armature assembly 212 is disposed inthe tube assembly 202 such that a shoulder 214A of the armature 214confronts a shoulder 208B of the pole piece 208. The sealing portion caninclude a closure member 216, e.g., a spherical valve element, that ismoveable with respect to the seat 218 and its sealing surface 218A. Theclosure member 216 is movable between a closed configuration, as shownin FIG. 1, and an open configuration (not shown). In the closedconfiguration, the closure member 216 contiguously engages the sealingsurface 218A to prevent fluid flow through the opening. In the openconfiguration, the closure member 216 is spaced from the seat 218 topermit fluid flow through the opening. The armature assembly 212 mayalso include a separate intermediate portion 220 connecting theferro-magnetic or armature portion 214 to the closure member 216. Theintermediate portion or armature tube 220 may be attached to thearmature 214 and closure member 216 by weld beads 215, 217,respectively.

Surface treatments can be applied to at least one of the end portions208B and 214A to improve the armature's response, reduce wear on theimpact surfaces and variations in the working air gap between therespective end portions 208B and 214A. The surface treatments caninclude coating, plating or case-hardening. Coatings or platings caninclude, but are not limited to, hard chromium plating, nickel platingor keronite coating. Case hardening on the other hand, can include, butis not limited to, nitriding, carburizing, carbo-nitriding, cyaniding,heat, flame, spark or induction hardening.

Fuel flow through the armature assembly 212 can be provided by at leastone axially extending through-bore 214B and at least one apertures 220Athrough a wall of the armature assembly 212. The apertures 220A, whichcan be of any shape, are preferably non-circular, e.g., axiallyelongated, to facilitate the passage of gas bubbles. The apertures 220Aprovide fluid communication between the at least one through-bore 214Band the interior of the valve body 206. Thus, in the open configuration,fiel can be communicated from the through-bore 214B, through theapertures 220A and the interior of the valve body 206, around theclosure member 216, and through metering orifice openings of an orificedisk 222 into the engine (not shown).

As a further alternative, a two-piece armature having an armatureportion directly connected to a closure member can be utilized. Althoughboth the three-piece and the two-piece armature assemblies areinterchangeable, the three-piece armature assembly is preferable due toits ability to reduce magnetic flux leakage from the magnetic circuit ofthe fuel injector 100 according to the present invention. It should benoted that the armature tube 220 of the three-piece armature assemblycan be fabricated by various techniques, for example, a plate can berolled and its seams welded or a blank can be deep-drawn to form aseamless tube.

The seat 218 is secured at the second end of the tube assembly 202. Anorifice disk 222 can be used in connection with the seat 218 to provideat least one precisely sized and oriented orifice in order to obtain aparticular fuel spray pattern and targeting. The precisely sized andoriented orifice can be disposed on the center axis of the orifice disk222 or, preferably disposed off-axis, and oriented in any desirableangular configuration relative to one or more reference points on thefuel injector 100. It should be noted here that both the valve seat 218and orifice disk 222 are fixedly attached to the valve body 206 by knownconventional attachment techniques, including, for example, laserwelding, crimping, and friction welding or conventional welding. Theorifice disk 222 is preferably tack welded to the seat 218 in a fixedspatial orientation to provide the particular fuel spray pattern andtargeting of the fuel spray.

In the case of a spherical valve element providing the closure member216, the spherical valve element can be connected to the armatureassembly 212 at a diameter that is less than the diameter of thespherical valve element. Such a connection would be on side of thespherical valve element that is opposite contiguous contact with theseat 218. A lower armature assembly guide 224 can be disposed in thetube assembly 202, proximate the seat 218, and would slidingly engagethe diameter of the spherical valve element. The lower armature assemblyguide 224 can facilitate alignment of the armature assembly 212 alongthe longitudinal axis A-A.

A resilient member 226 is disposed in the tube assembly 202 and biasesthe armature assembly 212 toward the seat 218. A filter assembly 228comprising a filter 230 and a preload adjuster 232 is also disposed inthe tube assembly 202. The filter assembly 228 includes a first filterassembly end 228A and a second filter assembly end 228B. The filter 230is disposed at one end of the filter assembly 228 and also locatedproximate to the first end 200A of the tube assembly 202 and apart fromthe resilient member 226 while the preload adjuster 232 is disposedgenerally proximate to the second end of the tube assembly 202. Thepreload adjuster 232 engages the resilient member 226 and adjusts thebiasing force of the member 226 with respect to the tube assembly 202.In particular, the preload adjuster 232 provides a reaction memberagainst which the resilient member 226 reacts in order to close theinjector valve 100 when the power group subassembly 300 is de-energized.The position of the preload adjuster 232 can be retained with respect tothe inlet tube 204 by an interference press-fit between an outer surfaceof the preload adjuster 232 and an inner surface of the tube assembly202. Thus, the position of the preload adjuster 232 with respect to theinlet tube 204 can be used to set a predetermined dynamic characteristicof the armature assembly 212.

The valve group subassembly 200 can be assembled as follows. Thenon-magnetic shell 210 is connected to the inlet tube 204 via the polepiece 208, and to the valve body 206. The non-magnetic shell 210 andpole piece 208 are joined by the weld bead 281. Assembly of thenon-magnetic shell may be performed using laser welding techniques asdescribed in more detail below with reference to FIGS. 2-4.

The filter assembly 228 is inserted along the axis A-A from the firstend 200A of the tube assembly 202. Next, the resilient member 226 andthe armature assembly 212 (which was previously assembled) are insertedalong the axis A-A from the injector outlet end 200B of the valve body206. The adjusting tube 232, the filter assembly 228 can be insertedinto the inlet tube 204 to a predetermined distance so as to permit theadjusting tube 232 to preload the resilient member 226. Positioning ofthe filter assembly 228, and hence the adjusting tube 232 with respectto the inlet tube 204 can be used to adjust the dynamic properties ofthe resilient member 226, e.g., so as to ensure that the armatureassembly 212 does not float or bounce during injection pulses. The seat218 and orifice disk 222 are then inserted along the axis A-A from thesecond valve body end 206B of the valve body 206. The seat 218 andorifice disk 222 can be fixedly attached to one another or to the valvebody 206 by known attachment techniques such as laser welding, crimping,friction welding, conventional welding, etc. Other preferred variationsof the valve group subassembly 200 are described and illustrated in U.S.Patent Publication No. 20020047054 published on Apr. 25, 2002, which ishereby incorporated by reference in its entirety.

The power group subassembly 300 comprises an electromagnetic coil 302,at least one terminal 304, a coil housing 306, and an overmold 308. Theelectromagnetic coil 302 comprises a wire that that can be wound on abobbin 314 and electrically connected to electrical contacts 316 on thebobbin 314. When energized, the coil 302 generates magnetic flux thatmoves the armature assembly 212 toward the open configuration, therebyallowing the fuel to flow through the opening. De-energizing theelectromagnetic coil 302 allows the resilient member 226 to return thearmature assembly 212 to the closed configuration, thereby shutting offthe fuel flow. The housing, which provides a return path for themagnetic flux, generally includes a ferro-magnetic cylinder surroundingthe electromagnetic coil 302 and a flux washer 318 extending from thecylinder toward the axis A-A. The flux washer 318 can be integrallyformed with or separately attached to the cylinder. The coil housing 306can include holes, slots, or other features to break-up eddy currentsthat can occur when the coil 302 is energized.

The overmold 308 maintains the relative orientation and position of theelectromagnetic coil 302, the at least one terminal 304, and the coilhousing 306. The overmold 308 includes an electrical harness connector320 portion in which a portion of the terminal 304 is exposed. Theterminal 304 and the electrical harness connector 320 portion can engagea mating connector, e.g., part of a vehicle wiring harness (not shown),to facilitate connecting the injector 100 to an electrical power supply(not shown) for energizing the electromagnetic coil 302.

According to a preferred embodiment, the magnetic flux generated by theelectromagnetic coil 302 flows in a circuit that includes the pole piece208, the armature assembly 212, the valve body 206, the coil housing306, and the flux washer 318. The magnetic flux moves across a parasiticairgap between the homogeneous material of the magnetic portion orarmature 214 and the valve body 206 into the armature assembly 212 andacross a working air gap between end portions 208B and 214A towards thepole piece 208, thereby lifting the closure member 216 away from theseat 218.

To set the lift, i.e., ensure the proper injector lift distance, severaltechniques may be utilized. According to a preferred technique, a liftsleeve 234 is displaced axially within the valve body 206. The positionof the lift sleeve 234 is adjusted by moving the lift sleeve 234axially. The lift distance is measured with a test probe (not shown).Once the desired lift is reached, the sleeve is welded to the valve body206, e.g., by laser welding. The valve body 206 is then attached to theinlet tube 204 assembly by a weld, preferably a laser weld. Theassembled fuel group subassembly 200 is then tested, e.g., for leakage.

The preparation of the power group sub-assembly 300, which may include(a) the coil housing 306, (b) the bobbin assembly including theterminals 304, (c) the flux washer 318, and (d) the overmold 308, can beperformed separately from the fuel group subassembly.

According to a preferred embodiment, wire is wound onto a pre-formedbobbin 314 having electrical connector portions 316 to form a bobbinassembly. The bobbin assembly is inserted into a pre-formed coil housing306. To provide a return path for the magnetic flux between the polepiece 208 and the coil housing 306, flux washer 318 is mounted on thebobbin assembly. A pre-bent terminal 304 having axially extendingconnector portions are coupled to the electrical contact portions 316 ofthe coil and brazed, soldered welded, or, preferably, resistance welded.The partially assembled power group assembly is now placed into a mold(not shown). By virtue of its pre-bent shape, the terminals 304 will bepositioned in the proper orientation with the harness connector 320 whena polymer is poured or injected into the mold. Alternatively, twoseparate molds (not shown) can be used to form a two-piece overmold asdescribed earlier. Additionally, a portion of the coil housing 306 canextend axially beyond an end of the overmold 308 to allow the injectorto accommodate different length injector tips. The extended portion maybe formed with a flange 306A to retain a sealing member such as theO-ring 10.

The assembled power group subassembly 300 can be mounted on a test standto determine the solenoid's pull force, coil resistance and the drop involtage as the solenoid is saturated during energization of the coil.

The inserting of the fuel group subassembly 200 into the power groupsubassembly 300 operation can involve setting the relative rotationalorientation of fuel group subassembly 200 with respect to the powergroup subassembly 300. According to the preferred embodiments, the fuelgroup and the power group subassemblies can be rotated such that theincluded angle between the reference point(s) on the orifice disk 222(including opening(s) thereon) and a reference point on the injectorharness connector 320 are within a predetermined angle. The relativeorientation can be set using robotic cameras or computerized imagingdevices to look at respective predetermined reference points on thesubassemblies, calculate the angular rotation necessary for alignment,orient the subassemblies and then check with another look and so onuntil the subassemblies are properly oriented. Once the desiredorientation is achieved, the subassemblies are inserted together. Theinserting operation can be accomplished by one of two methods:“top-down” or “bottom-up.” According to the former, the power groupsubassembly 300 is slid downward from the top of the fuel groupsubassembly 200, and according to the latter, the power groupsubassembly 300 is slid upward from the bottom of the fuel groupsubassembly 200. In situations where the inlet tube 204 assemblyincludes a flared first end, bottom-up method is required. Also in thosesituations, the O-ring 10 that is retained by the flared first end canbe positioned around the power group subassembly 300 prior to slidingthe fuel group subassembly 200 into the power group subassembly 300.After inserting the fuel group subassembly 200 into the power groupsubassembly 300, those two subassemblies are affixed together, e.g., bywelding, such as laser welding. According to a preferred embodiment, theovermold 308 includes an opening 308A that exposes a portion of the coilhousing 306. This opening 308A provides access for a welding implementto weld the coil housing 306 with respect to the valve body 206. Ofcourse, other methods or affixing the subassemblies with respect to oneanother can be used. Finally, the O-ring 10 at either end of the fuelinjector can be installed.

In operation, the electromagnetic coil 302 is energized, therebygenerating magnetic flux in the magnetic circuit. The magnetic fluxmoves armature assembly 212 (along the axis A-A, according to apreferred embodiment) towards the integral pole piece 208, closing theworking air gap. That movement of the armature assembly 212 separatesthe closure member 216 from the seat 218 and allows fuel to flow fromthe fuel rail (not shown), through the inlet tube 204, the through-bore214B, the apertures 220A and the valve body 206, between the seat 218and the closure member 216, through the opening, and finally through theorifice disk 222 into the internal combustion engine (not shown). Whenthe electromagnetic coil 302 is de-energized, the armature assembly 212is moved by the bias of the resilient member 226 to contiguously engagethe closure member 216 with the seat 218, and thereby prevent fuel flowthrough the injector 100.

Referring now to FIG. 2, a pole piece assembly 280 includes the polepiece 208 and the non-magnetic shell 210. The pole piece 208 includes anexternal, outwardly facing annular surface 286. The non-magnetic shell210 has an internal, inwardly-facing annular surface 285. The pole piece208 and the non-magnetic shell 210 are initially assembled by pressingthe two components together along the longitudinal axis A-A. Theexternal annular surface 286 has a diameter slightly greater than theinternal annular surface 285, resulting in a press fit or interferencefit. The relative diameters are controlled closely to control the pressforce required to assemble the parts, to avoid galling and other damageto the parts. A lubricant such as oil may also be used to amelioratethose problems. In an exemplary embodiment, the nominal diameter of theexternal surface 286 of the pole piece 208 is 6.39 mm.

A weld bead 281 connects the pole piece 208 and non-magnetic shell toform the assembly. The weld bead 281 is annular; i.e., it extends in aring-like manner around the joined components. The weld bead, as thatterm is used herein, is a mass of material originating from bothcomponents that are joined by the weld. The material has been liquefiedby energy from a welding energy source such as a laser beam (shownschematically at 299). The materials from the joined components arecomingled to some extent, although in most cases not completely. Theliquefied material is allowed to cool back to the solid phase. In acompleted assembly, the weld bead 281 is distinguishable from the parentparts because of its appearance, crystalline structure and othermetallurgical characteristics.

The present invention addresses the problem of “blow-out” of the weldbead 281. It is believed that “blow-out” is caused by gases trapped insmall voids or pockets between the surfaces 285, 286. For example,heated air or vaporized lubricant trapped in imperfections on thesurfaces may cause a differential pressure across the molten weld bead281.

As best shown in FIG. 3, a small step 290 is formed in either component208, 210. The step 290 may be formed in a machining operation such as agrinding or turning operation, or may be formed by a die in a pressoperation.

When the components 208, 210 are assembled and the surfaces 285, 286 areoverlapped, an annular press-fit region 292 is created, wherein the twosurfaces 285, 286 interfere, locking the parts together and creating asubstantially air-tight seal. A total length (not shown) of thepress-fit region is preferably about 1.3 mm.

The step 290 results in a non-contact region 291 of the surfaces 285,286. A gap 289 between the surfaces 285, 286 in the non-contact regionhas a preferred width 288 of between 0.005 and 0.025 mm.

The annular weld bead 281 formed by the laser beam 299 joins theexternal surface 286 with the internal surface 285 in the non-contactregion 291. The weld bead 281 forms a hermetic seal preventing liquidsand gases from passing between the components 208, 210. In a preferredembodiment, a nominal longitudinal distance 287 from a centerline 293 ofthe weld bead 281 to the press-fit region 292 is 1 mm. In otherembodiments, the distance 287 may be from 0.5 mm to 5.0 mm.

The weld bead 281 and the press-fit region 292 demarcate a substantiallysealed portion 295 of the gap 289. The sealed portion 295 issufficiently large so that the vapors inside the sealed portion havesufficient time and volume to minimize the pressure differential acrossthe weld bead 281 during the welding process. For example, the sealedregion may be approximately 0.75 mm in length in a longitudinaldirection. The sealed region may have a total volume of approximately0.037 mm³.

The inventors have found that a narrower gap is preferred (withoutbecoming a press fit) for reducing “blow-out” at the weld overlap. Forexample, the sealed region may have a ratio of length to width ofgreater than 10. Preferably, the ratio is about 50. It has beentheorized that as the gap width increases, vapor inside the sealedregion has more exposed area to the molten weld pool. The larger exposedarea to the weld pool leads to a more sudden increase in temperature,and consequently, vapor expansion and higher pressure. Conversely, asmall gap minimizes the rate at which the pressure increases due to thesmall exposed area to the molten pool.

In general, the longer the longitudinal length of the sealed portion295, the more resistant the design is to “blow-out” at the weld overlap.It has been theorized that the relatively cool facing walls of the twocomponents 208, 210 cool the expanding gas, slowing the rate at whichthe internal pressure increases.

The press-fit region 292 provides an effective seal for preventing weldslag and oxides created during the welding operation from entering thevalve body 206 (FIG. 1) and potentially contaminating the precisioncomponents contained there The press fit further is helpful in severalprocessing steps of the fuel injector, such as handling the assembly 280before welding.

FIG. 4 is a flow chart illustrating a method 400 according to oneembodiment of the invention. The method is for forming a fuel injectorhaving a fuel inlet, and fuel outlet and a fuel passageway extendingfrom the fuel inlet to the fuel outlet along a longitudinal axis. Themethod includes the step of constructing (step 410) a first fuelinjector component comprising a radially outwardly facing annularsurface. The first fuel injector component may, for example, be a polepiece. A second fuel injector component is also constructed (step 420),comprising a radially inwardly facing annular surface. The secondcomponent may be a non-magnetic shell.

At least one of the annular surfaces is shaped (step 430) to form, whenthe surfaces are overlapped, a non-contact region having a gap betweenthe surfaces, and a press-fit region where the first and second surfacesare in contact. The non-contact and press fit regions are adjacent. Theshaping step 430 may be done in combination with forming steps 410, 420,or may be done as a subsequent operation.

The first and second components are assembled (step 450) with theannular surfaces overlapping. Assembly is performed by aligning theparts along their longitudinal axes, and pressing the parts together toa predetermined length. The parts are in contact in the press-fitregion.

The annular surfaces are welded together (step 460) in the non-contactregion to form an annular weld bead, the weld bead and the contactregion bounding a substantially sealed portion of the gap. In apreferred embodiment, the welding operation is performed by rotating theassembled components about a longitudinal axis, and applying astationary laser beam in the non-contact region of the overlappingsurfaces. The laser is maintained “on” for slightly more than onerevolution of the assembly, to form the weld overlap. In one exemplaryembodiment, the assembly is rotated at 200 RPM. The laser is powered at850 Watts for 285 milliseconds, then at 820 Watts for 45 milliseconds.

The foregoing detailed description is to be understood as being in everyrespect illustrative and exemplary, but not restrictive, and the scopeof the invention disclosed herein is not to be determined from thedescription of the invention, but rather from the claims as interpretedaccording to the full breadth permitted by the patent laws. For example,while the method is disclosed herein with respect to tubular componentsof a fuel injector, the techniques and configurations of the inventionmay be applied to other tubular components where a hermetic weld isrequired. It is to be understood that the embodiments shown anddescribed herein are only illustrative of the principles of the presentinvention and that various modifications may be implemented by thoseskilled in the art without departing from the scope and spirit of theinvention.

1. A method for forming a fuel injector having a fuel inlet, and fuel outlet and a fuel passageway extending from the fuel inlet to the fuel outlet along a longitudinal axis, the method comprising the steps of: constructing a first fuel injector component comprising a radially outwardly facing annular surface; constructing a second fuel injector component comprising a radially inwardly facing annular surface; shaping at least one of the annular surfaces to form, when the surfaces overlap, a non-contact region having a gap between the surfaces, and a press-fit region where the first and second surfaces are in contact, the non-contact and press fit regions being adjacent; assembling the first and second components by overlapping the annular surfaces; and welding the annular surfaces together in the non-contact region to form an annular weld bead, the weld bead and the press-fit region bounding a substantially sealed and remaining portion of the gap.
 2. The method of claim 1, wherein the surfaces in the non-contact region are between 0.005 and 0.025 mm apart.
 3. The method of claim 1, wherein a center of the weld bead is at least 1 mm from the contact region.
 4. The method of claim 1, wherein the first fuel injector component is a pole piece and the second fuel injector component is a non-magnetic shell.
 5. The method of claim 1, wherein the step of welding the annular surfaces together comprises laser welding.
 6. The method of claim 5, wherein the step of laser welding further comprises the steps of: rotating the assembled first and second components about a longitudinal axis; and applying a laser welding beam.
 7. The method of claim 6, wherein the step of laser welding further comprises the steps of: rotating the assembled first and second components at 200 RPM; and applying a laser welding beam for 285 milliseconds at 850 Watts, then applying the laser welding beam for 45 milliseconds at 820 Watts.
 8. The method of claim 1, wherein the shaping step forms the non-contact region on a longitudinal side of the press-fit region opposite a valve body of the fuel injector.
 9. A method for assembling a pole piece and a non-magnetic shell of a fuel injector, the pole piece comprising a radially outwardly facing annular surface and the non-magnetic shell comprising a radially inwardly facing annular surface, the method comprising the steps of: forming an annular relief region on at least one of the annular surfaces; assembling the non-magnetic shell and the pole piece with the annular surfaces overlapping; the first and second surfaces being in contact in a press-fit region, and the first and second surfaces defining a gap therebetween at the relief region; welding an annular weld bead in the relief region, the weld bead joining the annular surfaces, the weld bead and the press-fit region bounding a substantially sealed and remaining portion of the gap.
 10. The method of claim 9, wherein the sealed portion of the gap has a volume of at least 0.037 mm³.
 11. The method of claim 9, wherein the surfaces in the gap are between 0.005 and 0.025 mm apart.
 12. The method of claim 9, wherein a center of the weld bead is at least 1 mm from the press-fit region.
 13. The method of claim 9, wherein the step of welding a weld bead comprises laser welding.
 14. The method of claim 13, wherein the step of laser welding further comprises the steps of: rotating the assembled non-magnetic shell and pole piece about a longitudinal axis; and applying a laser welding beam.
 15. The method of claim 14, wherein the step of laser welding further comprises the steps of: rotating the assembled non-magnetic shell and pole piece at 200 RPM; and applying a laser welding beam for 285 milliseconds at 850 Watts, then applying the laser welding beam for 45 milliseconds at 820 Watts. 